ISIS militants have seized control of every major border crossing from Syria into Iraq — as well as four critical towns in Iraq’s western province.

The radical group now controls large areas of both countries, and there are concerns that the Iraqi government could collapse.

The four towns fell in close succession, opening the Syrian border and areas in Syria controlled by ISIS (Islamic State of Iraq and Syria) to areas in western Iraq also held by the group.

After a lightning offensive that began in the northern city of Mosul — Iraq’s second-largest city — most of central and northern Iraq’s Sunni Muslim-populated areas fell in 10 days. The militants have now refocused their efforts on western Iraq.

Defenders: A show of force to warn off ISIS. Armed Iraqi Shiite militiamen, the followers of Shiite cleric Muqtada al-Sadr, parade in the oil rich province of Kirkuk, northern Iraq.Source:AP

Iraqi armed Shiite militiamen during a parade in the northern oil rich province of Kirkuk, Iraq.Source:AP

On Saturday evening, a large force of ISIS fighters came out of the Syrian desert to attack the last major border crossing in Iraq government hands at the town of Qaim, taking control of it late Saturday night in what local residents described as a vicious battle that destroyed the government forces in the area.

Militants stand with a captured Iraqi Army Humvee at a checkpoint some 250 kilometres north of Baghdad, Iraq.Source:AP

Meanwhile, jihadists fighting in Syria’s war on Sunday put to use for the first time American-made Humvees that they seized during a lightning offensive in Iraq this month, a monitor said.

ISIS used the armoured vehicles to capture the villages of Eksar and Maalal in Aleppo province, which borders Turkey, said the Syrian Observatory for Human Rights.

It came after heavy fighting against the Islamic Front and its al-Qaeda-affiliated ally, the Al-Nusra Front, said the Observatory, a Britain-based group that gets its information from a network of sources on the ground.

Peshmerga military direct traffic at a Kurdish Check point on June 14, 2014 in Kalak, Iraq. Picture: Getty Images

Smoke billows from an Iraqi oil refinery north of Baghdad after an ISIS attack. Picture: USGS/NASA/Getty ImagesSource:Getty Images

Young jihadi: A child is pictured on an ISIS-supporting Twitter feed wearing a balaclava and holding a sign saying 'God is greatest'.Source:Supplied

The two villages are located near the town of Azaz, which ISIS militiamen abandoned at the end of February under attack from rebels fighting to oust Syria’s President Bashar al-Assad.

ISIS, which espouses a radical interpretation of Islam and aims to set up a state stretching across the Syria-Iraq border, is now expected to launch a bid to retake Azaz.

ISIS seized the Humvees and sent them to Syria after Iraqi soldiers abandoned them during a surprise Sunni jihadist offensive that claimed Iraq’s second city of Mosul and swathes of other territory in mid-June.

Terror: Refugee families huddle at a relative’s house after they fled from the Shiite village of Beshir in northern Iraq.Source:AP

Also on Sunday, ISIS gunmen abducted 20 Kurdish students on the road between Hasakeh and Qamishli in north-eastern Syria, said the Observatory.

It comes three weeks after ISIS kidnapped 145 Kurdish students in Aleppo, as well as 193 Kurdish civilians at Qabasine village in the same province.

Parents of five students who managed to escape said the jihadists demanded that they join them in the fighting.

Kurdish militias, who are also trying to expand their autonomous region, have fought for months with ISIS, which has been seeking to seize from their control oilfields in northern and eastern areas.